This invention relates generally to a gas transfer system in which a gas or other compressible fluid is dissolved in water or other relatively-incompressible fluent material to be treated, and more particularly to a system in which the fluent material to be treated is subjected to pressure to enhance the absorption of gas with a minimal expenditure of energy.
Though there are many industrial processes which call for the transfer of a gas to a fluent material, by way of example we shall consider a process in which the fluent material is waste water or sewage and the gas is oxygen. In waste water treatment, dissolved oxygen is the factor that determines whether biological changes are brought about by aerobic or anaerobic organisms. The former requires free oxygen and produces innocuous end products, whereas the latter can utilize chemically-bonded oxygen and the end products are generally obnoxious. Hence a vital unit operation in waste treatment involves the transfer of oxygen to the liquid for the creation and maintenance of aerobic conditions.
The productivity of a gas transfer system in which oxygen is dissolved in water waste is determined by the gas transfer rate. This productivity may be significantly raised by pressurizing the liquid, thereby enhancing absorption of the oxygen by increasing the gas saturation value of the liquid. Normally such pressurization entails the expenditure of a considerable amount of energy in order to maintain a flow of liquid waste through the gas transfer reactor under high pressure conditions. The energy required to pump the liquid is quite high because of the back pressure needed to dissolve the oxygen.
With rising energy costs, conventional pressurized gas transfer systems are becoming much more expensive to operate. This factor, in a period of restricted municipal budgets, may in some instances discourage the acquisition of a modern sewage treatment system by a municipality despite the pressing need for pollution control.
The patent to Speece, U.S. Pat. No. 3,926,588, discloses a gas transfer system which achieves a high level of pressurization to enhance the absorption of oxygen or other gas by fluent material without, however, entailing the high expenditure of energy normally required to effect such pressurization. In the Speece system, the liquid to be treated is alternately supplied to two plug flow chambers against a relatively low back pressure sufficient to displace gas-treated liquid therefrom. Gas is dissolved in the liquid in each flow chamber while it is sealed from the influent source, such sealing being effected by the closing of valves during each operating cycle of the system.
When a given flow chamber is sealed from the influent source, it is connected into a closed flow loop which includes a gas transfer apparatus. A circulatory pump induces recirculating flow in the flow loop, while a pressurizing pump maintains the liquid being recirculated through the gas transfer apparatus under continuous pressure. Valves are provided to isolate the pressurized loop from the liquid inflow and outflow of the system, the various valves being operated in a predetermined sequence in the course of an operating cycle.
The Speece system is both elaborate and sizable, for it involves two distinct plug flow chambers, several valves, programmed controls for operating the valves in proper sequence as well as a complicated network of pipes to intercouple the valves with the chambers, the pumps and the gas transfer device.
Hence while the gas transfer arrangement disclosed in the Speece patent makes it possible to minimize the energy requirements for such systems, the advantages gained thereby are offset to a significant degree by the large space requirements of this arrangement, its complexity and its relatively high construction, installation and maintenance costs.